A protein carboxyl methyltransferase (EC 2.1.1.77) that recognizes age-damaged proteins for potential repair or degradation reactions has been found in all vertebrate tissues and cells examined to date. This enzyme catalyzes the transfer of methyl groups from S-adenosylmethionine to the carboxyl groups of D-aspartyl or L-isoaspartyl residues that are formed spontaneously from normal L-aspartyl and L-asparaginyl residues. A similar methyltransferase has been found in two bacterial species, Escherichia coli and Salmonella typhimurium, suggesting that this enzyme performs an essential function in all cells. In this study, we show that this enzyme is present in cytosolic extracts of six additional members of the a and y subdivisions of the purple bacteria: Pseudomonas aeruginosa (,y), Rhodobacter sphaeroides (a), and the 'y enteric species Klebsiella pneumoniae, Enterobacter aerogenes, Proteus vulgaris, and Serratia marcescens. DNA probes from the E. coli methyltransferase gene hybridized only to the chromosomal DNA of the enteric species. Interestingly, no activity was found in the plant pathogen Erwinia chrysanthemi, a member of the enteric family, nor in Rhizobium meliloti or Rhodopseudomonas palustris, two members of the a subdivision. Additionally, we could not detect activity in the four gram-positive species Bacillus subtilis, B. stearothermophilus, Lactobacillus casei, and Streptomyces griseus. The absence of enzyme activity was not due to the presence of inhibitors in the extracts. These results suggest that many cells may not have the enzymatic machinery to recognize abnormal aspartyl residues by methylation reactions. Since the nonenzymatic degradation reactions that generate these residues occur in all cells, other pathways may be present in nature to ensure that these types of altered proteins do not accumulate and interfere with normal cellular physiology.In all vertebrate tissues examined to date, a cytosolic L-isoaspartyl/D-aspartyl protein carboxyl methyltransferase (EC 2.1.1.77) catalyzes the transfer of the methyl group from S-adenosylmethionine to L-isoaspartyl or D-aspartyl residues (3,4,14,25,27). These unusual residues can accumulate in all proteins via the spontaneous decomposition of normal L-aspartyl and L-asparaginyl residues (11, 33) and can inhibit the function of the modified proteins and/or the normal pathways for their enzymatic degradation (12,16,19,22). The enzymatic methylation of these altered residues in vitro can lead to their conversion to L-aspartyl residues via a nonenzymatic repair pathway (9,16,17,21).A similar methyltransferase, characterized by its ability to methylate L-isoaspartyl-containing peptides, has also been detected in the cytosolic fraction of procaryotic cells such as Salmonella typhimurium (26) and Escherichia coli (8). The enzyme from the latter source has been purified, and the nucleotide sequence and chromosomal location of its structural gene pcm (59 min) have been determined (8). The deduced amino acid sequence of the E. coli enzyme is 31% identical to t...